Specially designed threaded connections for interlocking oilfield tubular members are commonly used during production and drilling operations in the petroleum industry. The threaded connections usually consist of a male pin member which makes up into a female box member, but may alternatively employ a coupling which is threaded at both ends for interlocking two tubular members. The interlocking tubular members when manipulated downhole within drilling and/or production operations are subjected to various tensile, compressive and shear forces resulting in fatigue and ultimate failure of the weaker threaded connection. Therefore, various specially designed threads for improving fatigue resistance have been used in the prior art for both drilling applications (drill pipe and tool joints) and production applications (marine risers and casing).
Among the most important characteristics for downhole tubulars is the mechanical ability of the threaded connections to resist tension, compression and bending loads. Other important characteristics include dependable pressure sealing capabilities from either inside or outside the tubular, regardless of mechanical loading, and special geometries such that the connection exterior surface is flush with either or both the outside diameter and inside diameter of the tubular. Downhole tubular connections used in completion operations are not ordinarily subjected to cyclic loading which can cause fatigue and therefore, are not typically designed to resist such loading. Tubular connections used in completion operations frequently utilize square or four-sided thread forms which resist mechanical loading while requiring a minimum of space and material. These thread forms normally incorporate sharp radii between intersecting thread faces that may work well for some downhole completion operations, but act as concentrated stress points which can initiate fatigue cracking when the tubular string is moved through a load in the well bore.
Drill pipe is normally used to drill wells by rotation, making fatigue resistance important. The evolution of fatigue resistant drill pipe has resulted in drill pipe connectors that are significantly greater in diameter than the pipe body, generally making these connections impracticable and uneconomical for downhole completion operations. Drill pipe connections are also designed with very little pressure sealing capability.
Consequently, risers have been used over the years to accommodate fatigue resistant connections in downhole completion applications, such as deep water drilling. Risers are protective casing strings that can experience mechanical loading from the water and from the rig, and provide dependable pressure sealing. Risers are normally subjected to fatigue loading situations and, like drill pipe connections, must be resistant to fatigue or cyclic loading.
Thread designs for tool joints are typically restricted by their application to drilling operations, which makes their use in production risers and related applications counter-productive to improving fatigue resistance. For example, U.S. Pat. No. 4,549,754 to Saunders et al. discloses a threaded tool joint for reducing stress and improving fatigue resistance. However, since the tool joint distributes uneven loads, the result is more stress on the threads farthest from the pin shoulder.
U.S. Pat. No. 4,708,513 to Roche et al. discloses an extended lift coupling for joining tubular members in end-to-end relationship for improved fatigue resistance caused by applied cyclic tensile forces. An external compressive axial preload places a torus on a pin in axial compression. Make-up of a lock ring maintains the compressive strain on the torus and the lock ring when the axial make-up force is removed. Increased tensile loading is partially absorbed in benign compressive stress by the locking ring, which does not improve fatigue resistance caused by radial bending of the tubular members. Nevertheless, downhole connections offer most of the characteristics designed for risers, such as pressure integrity, resistance to high mechanical loads and economical costs, but not fatigue resistance. Therefore, to meet various riser application needs, most riser systems have used expensive connections manufactured on heavy wall material, which is then welded to the pipe, and result in a very expensive solution to solving the fatigue resistance problem.
The disadvantages of the prior art are overcome by the present invention and a threaded connection is hereinafter disclosed for use in oilfield tubulars, and in particular in risers, with improved fatigue resistance.